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ON  THE  COLOURS  OF THICK PLATES.                     191
the term. When polarized light is scattered, as for example when it is reflected from or transmitted through white paper, it loses its polarization, but when polarized light suffers regular diffraction it retains its polarization.
Having placed a small flame near the centre of curvature of a concave mirror, of which the surface had been prepared with milk and water, I placed a Nicol's prism close to the flame, so as to polarize the light incident on the mirror. On examining the rings with another Nicol's prism, they proved to be perfectly polarized.
33. It may not be considered out of place here to point out what appears to be the cause of a phenomenon observed by M. Pouillet. In an experiment in which rings were occasioned simply by the straight edge of an opaque body held in front of a metallic speculum, it was found that they were formed distinctly in only one half of their circumference. The reason of this appears to be simply as follows. As the waves of light pass the diffracting edge in their progress towards the mirror, those rays which are diffracted inwards, so as to enter the geometrical shadow, after being regularly reflected at the mirror fall upon the opaque body, by which they are stopped. As these rays are required for the formation of that half of the system which lies on the same side as the opaque body, the other half only is well formed. The first half may be formed obscurely by a few rays which are diffracted in the required direction at such a distance from the edge that on their return they pass clear of the edge, and so proceed to interfere with other rays diffracted by the edge on the return of the general wave.
SECTION VI. Investigation of the angles of diffraction.
34. Something yet remains to be done in order to complete the theory of these rings and bands, namely, to compare the two diffractions which a wave of light experiences at its entrance into the glass and on its return, respectively. For the phase and intensity of a ray diffracted in a given direction depend altogether on the circumstances under which the diffraction takes place; and